Gap junction (GJ) mediated electrical synaptic transmission is considered an essential form of interneuronal communication. It critically contributes to important functional processes in diverse regions of the mammalian CNS and has been linked to a variety of neurological conditions. Plasticity of electrical synapses underlies important functions by reconfiguring networks of electrically coupled neurons, whose disruption might contribute to neurological dysfunction. In contrast to chemical synapses, less is known regarding the molecular mechanisms that regulate the strength of electrical synapses. This proposal focuses on understanding mechanisms underlying plastic changes in GJ communication observed at mixed, electrical and chemical, synapses that couple primary auditory afferents to the teleost Mauthner (M-) cells, at which GJs are formed by fish homologs of the widespread mammalian GJ protein connexin36 (Cx36) and where it is possible to analyze cellular and sub-cellular mechanisms in-vivo. Our studies in goldfish show that both components of the mixed synaptic response undergo activity-dependent potentiation of their respective strengths. Remarkably, our recent findings indicate that factors regulating the turnover and number of functional GJ channels might constitute major determinants of the strength of electrical transmission. We propose here to investigate the contribution of trafficking of GJ channels as a possible mechanism for regulating the strength of electrical transmission. For this purpose, we will take this unique model mixed synapse to a new level of analysis by investigating their properties in larval zebrafish, whose transparency will make it possible to track individual molecules within living cells, in vivo. Supporting this possibility, our preliminay results indicate that mixed synapses in larval zebrafish are molecularly and functionally analogous to those of adult goldfish. The proposal has two aims: Aim 1 is to generate transgenic zebrafish in which neuronal gap junction proteins are tagged with fluorescent proteins, and Aim 2 is to investigate the turnover of fluorescently tagged gap junction channels in-vivo and its properties under conditions that trigger plasticity. The amenability of zebrafish larvae to image the movement of fluorescently tagged GJ channels in-vivo should permit the monitoring of active synapses undergoing plasticity providing an unprecedented window for the analysis of this modality of transmission at which detailed molecular mechanisms could be investigated combining electrophysiology and live imaging with powerful genetic manipulations. Thus, the development of this zebrafish model will provide a new powerful tool to study molecular aspects of Cx36-mediated synapses (prevalent in mammals) that could lead to the identification of novel therapeutic opportunities for the treatment of various neurological conditions.